Imaging lenses with ultra wide field of view (with full field of view>=100°up to 180° FOV fisheye lenses) are required for many applications. Fisheye lenses are also commonly used for security and surveillance applications. New emerging applications include back-up cameras for cars, and interior monitoring cameras for buses and airplanes. Prior art designs of wide-angle lenses tend to have a significant amount of optical distortion. Examples of similar related inventions include U.S. Pat. No. 7,023,628 for a Compact Fisheye Objective Lens issued to Alex Ning on Apr. 4, 2006 and U.S. Pat. No. 6,844,991 for a Fisheye Lens issued to Keiko Mizuguchi on Jan. 18, 2005.
The low light performance of an FOV fisheye objective lens is important. The relative aperture of the lens must also be high. A lens relative aperture (also known as f/# of the lens) of less than or equal to 3 is generally preferred. There is also a need for small size and low weight for many emerging applications. The image quality of the lens must also be high for compatibility with mega pixel class electronic imagers.
Optical distortion of any lens can be characterized by its image height vs. field angle. The off-axis image height h is defined as the distance between the interception of an off-axis chief ray with the image plane, and the optical axis. The field angle is the angle θ between the off-axis chief ray and the optical axis. The image plane is defined as the best focal plane when the object is at infinite distance from the lens. FIG. 1 illustrates the definitions of the terms used.
Prior art fisheye lenses such as the DF1.8HB marketed by Fujinon in Japan use a large number of lens elements to obtain the performance requirements of a wide field of view, a large aperture and high image quality. However, the lens assembly is large, heavy and expensive to produce because of the large number of elements used. Additional examples of prior art fisheye lens designs are taught in U.S. Pat. Nos. 3,589,798, 3,597,049, 3,734,600, 3,737,214, 3,741,630, 4,412,726, 6,844,991, and also in JP Patents 63-017421, and 60-153018. The number of lens elements in the lenses taught by these references range from 8 to 12 elements.
The image height “h” that is produced on a focal or image plane by rays processed from an object to the pupil of an objective lens and then to an image plane is a function of field angle θ where θ is the angle that a chief ray makes with the optical axis of the lens and where the chief ray is propagating from an extreme point on the object to the center of the lens pupil. In a distortion free lens (also known as a rectilinear leans), the height of the image on the image plane is a function of the angle θ as characterized below in Equation 1.h(θ)=f*tan(θ)  Eq. 1
In Equation 1, the parameter “f” represents the effective focal length (EFL) of the lens. Most standard photographic lenses follow this relationship. However, as the field of view required increases, it becomes more difficult or impossible to design lenses that satisfy Equation 1. Objective or fisheye lenses that capture an ultra wide-angle image, are better characterized by Equation 2 as follows:h(θ)=f*(θ)  Eq. 2
The angle θ in Equation 2 is given in radians. If a lens satisfies Equation 2, the lens is referred to as an “f-θ” lens. It should be understood that an image formed by a perfect “f-θ” lens will also be distorted. The image height “h” obtained by the above listed fisheye lenses including the image produced by the objective lens of U.S. Pat. No. 7,023,628 for a Compact fisheye objective lens that issued to Alex Ning, the common inventor on Apr. 4, 2006) satisfy Equation 3, below:h(θ)<f*(θ) for θ>0  Eq.3